基于辐射自显影胶片EBT2的剂量验证系统剂量响应特征及稳定性分析

[目的]分析不同扫描光源下辐射自显影EBT2胶片的剂量响应特性及IMRT剂量验证系统的稳定性.[方法]使用6MV光子线照射EBT2胶片,分析不同扫描光源下胶片的剂量响应特性;然后通过对扫描仪的扫描不确定性和扫描方向对扫描仪读数的影响进行测试以评估由EBT2胶片、Uniscan C880平板扫描仪和FilmQA剂量分析软件组成的IMRT剂量验证系统的性能.[结果]EBT2胶片在红光扫描光源下的剂量响应敏感性最高且其响应曲线是非线性的,使用三次多项式可以满足该剂量响应曲线的拟合要求.扫描仪的单次扫描不确定性和多次扫描不确定性分别是0.22％和1.12％,胶片放置方位对扫描仪读数影响很大.[结论]基于辐射自显影胶片EBT2的剂量响应敏感性随剂量上升而下降,不同光源下剂量响应敏感性不同.基于EBT2的调强放疗剂量验证系统操作简单、稳定性好,值得在临床中推广应用.     

冲洗时间因素对胶片剂量的影响

目的监测冲洗药水更换周期内HS-126E X线胶片自动冲洗机的性能稳定性，研究冲洗时间因素对胶片剂量测量的影响。方法相同条件下曝光来自同一批的15张Kodak X—Omat—V胶片，随机分成5个组，每组3张。在冲洗药水更换周期的5个工作日内，每天冲洗1组。用RIT113胶片剂量测量系统软件测量每张胶片照射野中心的平均吸光度（A）值，进行组间比较。另取10张胶片，分成5个组，均以垂直法曝光，曝光后每天冲洗1组，用于建立校正文件并绘制感光度曲线。建立校正文件后，对整个冲洗药水更换周期内感光度曲线进行质量控制，并观察胶片敏感性的变化。再取来自同一批的胶片15张，分成5个组，均以平行法曝光，曝光后每天冲洗1组，用当天绘制的感光度曲线测量中心轴上1．5cm深度的点剂量，与相同条件下的电离室测量值比较，并进行组间比较。结果在冲洗药水更换周期内，各组胶片A值的均数有随时间延长降低的趋势，方差分析显示，各组A值的均数间差异均有统计学意义。感光度曲线的质量控制显示出随时间延长向下倾斜趋向，体现了胶片敏感性受冲洗因素影响而降低。各组胶片点剂量的差异〈2％，与电离室实测剂量的差别〈3％。冲洗周期内点剂量随时间的变化较小。结论HS-126E X线胶片自动冲洗机在冲洗药水更换周期的早期是稳定的。用于辐射剂量测量的胶片应当在冲洗药水更换周期内尽早冲洗，这对A值的测量尤其重要。尽管冲洗因素对感光度曲线影响显著，且胶片敏感性在冲洗药水更换周期内明显降低，但点剂量在冲洗药水更换周期内的变化较小。建议同时冲洗用于剂量测量的胶片和用于校正的胶片。     

对HT系统MVCT扫描图像CT值及噪声影响因素的研究

目的 探讨HT的MVCT扫描剂量率波动和不同扫描精度选择对图像CT值及噪声的影响。方法 MVCT在不同剂量率和扫描层厚下扫描带各种密度棒的水模体,对应各密度棒的CT值得出各自条件下的IVDT曲线进行对比。MVCT在不同剂量率和扫描层厚下扫描带有均匀密度棒的水模体,测量不同扫描条件下均匀水模体中的CT图像噪声。对结果进行方差及相关性分析。结果不同扫描剂量率下对CT值影响显著(P=0.000),各密度棒对应CT值的变化大小与密度呈正相关(R²=0.846),既高密度物质的CT值受的影响较大。不同剂量率对噪声也有显著影响,扫描剂量率越低图像噪声越大(P=0.000)。2、4、6 mm扫描层厚对CT值(P=1.000)及图像噪声(P=0.667)无显著影响。结论 MVCT剂量率改变会引起CT值及图像噪声改变,在临床中应定期对MVCT剂量率及IVDT曲线稳定性进行监测,以保证ART计划中MVCT图像剂量计算的准确。     

兆伏级锥形束CT影像质量定量分析

目的 定量分析不同成像条件下兆伏级锥形束CT(MVCBCT)的影像质量,为临床应用提供参考.方法 采用西门子ONCOR直线加速器上的MVCBCT设备,在不同的成像条件下扫描影像质昔模体.通过分析影像均匀性、噪声、空间分辨率、对比度分辨率及成像剂量来评估其影像质量,并与常规大孔径CT影像进行定量比较.结果 MVCBCT影像噪声随加速器出束跳数(MU)增加而减少.均匀性指数与成像MU数及重建矩阵无线件关系.空间分辨率上采用256×256重建矩阵除5MU条件下为0.25 lp/mm,其他皆为0.4 lp/mm.随MU数增加对比度分辨率增加.成像剂量上头颈患者接受最大剂量为1.2 cGy/MU,中心位置剂量为0.8～0.9 cGy/MU,腹部最大和中心处分别为1.3cGy/MU和0.7 cGy/MU.结论 MVCBCT的噪声、均匀性、空间分辨率及对比度分辨率都差于常规扇形束CT.通过选择恰当成像参数和重建参数,可在患者接受尽量低成像剂量的同时获得足够分辨率来分辨骨组织、空腔及部分软组织用于影像引导放疗.     

VMAT下肺等效均匀剂量预测RP价值研究

目的 对VMAT下基于肺等效均匀剂量的放射性肺炎预测价值的临床应用效果进行评价及最优化a值的搜寻。 方法 选取2015—2016年间接受过VMAT的 65例肺癌患者,根据放疗结束后有无RP分成两组,导出DVH等信息,用自编数值分析程序进行数据分析。分别计算a在-50~50区间两组肺等效均匀剂量值变化,找出两组相对肺等效均匀剂量值差距最大的a值。采用成组t检验对发生和未发生放射性肺炎的 V₅、V₂₀、V₃₀、MLD和肺等效均匀剂量(a_optimal)进行分析;采用Pearson相关分析法分析V_dose和肺等效均匀剂量(a_optimal)与RP之间关系;采用 Logistic回归方法建立疾病预测模型。 结果 a=0.3时发生和未发生组肺等效均匀剂量相对差取得最大值(627.94 cGy和510.23 cGy,相对剂量差 R=23.07%)。相对差R在-50~-5区间缓慢减少,在-5~0急剧增加且在a=0.3时取得最大值;在a从 0.3~4.0区间快速减小后直到研究终点50都趋于缓慢减小趋势。传统物理容积剂量阈值相关性分析也提示肺等效均匀剂量(a=0.3时)和 V₅、V₁₀、V₂₀、MLD有相关性(r=0.929,P<0.05)。 结论 采用VMAT技术进行胸部肿瘤放疗患者,肺等效均匀剂量(a=0.3时)能较好区分有无肺炎两组,建议肺等效均匀剂量限制在510 cGy内,肺等效均匀剂量和常规物理剂量相结合对非均匀照射条件下RP有一定临床预测价值。     

HD-V2胶片在超高剂量率电子线射束剂量测量中适用性的研究

目的评估Gafchromic HD-V2胶片在改造常规直线加速器超高剂量率电子线射束中剂量测量的适用性, 及其能量和剂量率响应特性。方法使用HD-V2胶片测定改造常规直线加速器电子线的平均剂量率, 测量的结果与改进型Markus平板电离室、丙氨酸剂量计进行比较, 并使用HD-V2胶片进行初步物理特性测量。利用直线加速器上不同能量射线(6 MV X线和9、16 MeV电子线), 剂量范围10～300 Gy, 研究HD-V2胶片的能量响应。同时利用直线加速器剂量率(0.03、0.06、0.1 Gy/s)及改造机超高剂量率(范围100～200 Gy/s)射束, 研究HD-V2胶片的剂量率响应。结果使用HD-V2胶片测量改造后电子线在源皮距(SSD)100 cm处平均剂量率约为121 Gy/s, 与改进型Markus平板电离室、丙氨酸剂量计测量结果一致。改造后超高剂量率射束百分深度剂量(PDD)曲线参数与常规9 MeV电子线相近, 离轴剂量分布总体呈现中心轴最高, 随离轴距离增加, 剂量逐渐下降的特点。对于不同能量6 MV X线和9、16 MeV电子线, 剂量范围20～300 Gy, HD-V...     

HT系统兆伏级CT图像质量和剂量计算研究

目的 定量对HT系统兆伏级CT图像质量及剂量计算准确性进行评估。方法 应用兆伏级CT扫描Cheese phantom模体,分别对图像成像几何精度、噪声、图像均匀性、空间分辨率、密度-CT值的转化及剂量计算准确性进行测试,并与常规千伏级CT进行对比分析。结果 兆伏级CT图像的成像几何精度在三维方向均<2 mm,MVCT图像噪声、均匀性、空间分辨率都差于千伏级CT。基于CT图像进行剂量重建的DVH均与千伏级CT计划的DVH有很好一致性。结论 兆伏级CT图像几何尺寸精确,成像剂量低,剂量计算精确,满足于临床要求。     

CBCT图像质量质控体系建立及结果分析

目的 通过建立完善的图像质控体系来保证设备满足临床需要。方法 对Catphan₅₀₄模体进行高质量头部、标准剂量头部、盆腔扫描并分析各自CBCT图像,检测CT值线性、CT值均匀性、空间分辨力、密度分辨力是否满足要求。采用成组t检验比较不同扫描条件的结果差异。结果采用标准剂量头部扫描条件得到的CT值更接近标准CT值,优于盆腔扫描条件得到的结果。CT值均匀性检测结果显示标准剂量头部优于高质量头部和盆腔(9.7±3.9∶17.9±5.3,P=0.00和9.5±4.0∶31.1±5.7,P=0.00)。密度分辨力高质量头部、盆腔扫描方式优于标准剂量头部(5.6±0.1∶1.3±0.5,P=0.00和6.0±1.0∶1.3±0.5,P=0.00)。空间线性距离结果十分准确,范围为4.98~5.06 cm。结论 设备空间线性距离准确、稳定,而CT值线性、CT值均匀性受扫描条件影响较大。对空间分辨力、密度分辨力则要做到因设备不同而设定标准与误差范围。     

CT模拟定位机的扫描参数对放疗计划系统剂量计算的影响

目的:探讨模拟定位CT的扫描参数对放射治疗计划系统剂量计算的影响。方法:在不同扫描条件下,将可拆分的带有人体组织等效密度插件的参考模体(CIRS-062)在同一台CT模拟定位机上进行扫描,比较得到的各密度插件CT值;将同一IMRT治疗计划移植到CT图像上,在相同条件下进行剂量计算,测量和分析模体内剂量变化。结果:不同扫描条件下同一模块的CT值有改变,CT值受管电压影响较大;扫描条件对剂量分布有一定影响,其中低剂量区域影响较高剂量区域大。结论:CT模拟定位机的扫描条件对放疗计划系统剂量计算有一定影响,CT模拟定位机的质量保证与质量控制也是放射治疗过程中的一个重要环节。     

放疗计划CT值的校准检测及其影响因素分析

背景与目的:放射治疗计划系统(treatmentplanningsystem,TPS)依赖CT扫描得到的CT值建立CT-密度转换曲线,并根据转换所得的组织密度(或电子密度)进行组织不均匀性剂量校正计算,从而得出放射治疗计划的剂量分布。同一组织在不同的扫描条件下得到的CT值可能产生相当大的差异,从而导致剂量计算的误差。本研究利用等效模体在不同条件下进行CT扫描,测量各因素对CT值影响的大小及分析各影响的相对严重程度。方法:用含有已知密度的不同组织替代材料组成的模体在不同的CT扫描条件下进行CT值测量,评价扫描参数、患者体形和CT床面等因素对CT值的影响。实验条件分为3组:(1)以不同的扫描电压、层厚及扫描方式(逐层/螺旋扫描)对相同模体进行CT扫描;(2)改变各不同密度的组织替代材料在模体中的位置和排列顺序,固定CT参数进行图像扫描;(3)扫描参数不变,分别将相同模体置于床面和悬空进行比较扫描和CT值测量。结果:扫描电压和床面分别对高密度组织和低密度组织的CT值影响较大,前者对皮质骨模型的CT值改变可达150Hu(12.7%),后者对水模的CT值改变达26.34%。模体几何位置对CT值的影响可以忽略(<3%)。结论:不同的CT扫描参数和定位条件可能使某一组织重建影像的CT值发生改变,从而造成TPS剂量计算误差。尤其以扫描电压和定位床面的散射对CT值的影响为甚。已知密度的不均匀体模可以用于放射治疗计划CT扫描的质量保证检测和CT值的校准。各放疗中心应根据实际测量和校准结果制定统一的CT定位扫描规范,对每一患者采用一致的扫描参数和定位条件,以保证治疗计划剂量计算的精度。     

Optimization of the Gafchromic™ EBT protocol for IMRT QA

Quality assurance of external beam (radio)therapy (EBT) requires tools with specific characteristics. A radiochromic film dubbed “Gafchromic? EBT” (G-EBT) that is particularly suited for external beam therapy because of its features was introduced in 2004. Its characteristics, especially the high spatial resolution, make it suitable for measurement of dose distributions in radiotherapy, especially intensity-modulated radiation therapy (IMRT). While several aspects of the film characteristics have been previously reported separately, we present a comprehensive evaluation centered on practical IMRT verification, leading to an optimized protocol. Therefore the constancy within one batch, the relationship between optical density (OD) and dose (dose range between 1.4Gy and 8.4Gy) and the dose rate dependence for four dose rates (55, 108, 217, 441 MU/min) were investigated. In addition to these characteristics, energy dependence between two energies (50 kV and 6 MV), tissue equivalency, post irradiation coloration over one month, pressure and temperature sensitivity were evaluated. We then optimized the protocol using the G-EBT films, in combination with an EPSON-Expression? 1680pro flatbed scanner, for IMRT QA, while either striving to keep the compound error as small as possible or trying to reduce evaluation time. As a basis for this protocol optimization, the characteristics of the scanner (such as inhomogeneity of the scanning field) and its software (such as consequences of extracting only the red color channel) had to be determined first. The interaction of film and scanner (variation of the OD depending on the scanning direction or the scanning resolution) was assessed as well. Using the optimized protocol for IMRT QA, the compound error could be reduced to approximately 2% for a quality-driven approach and maximum 5.5% for an approach attempting to reduce procedure time. While the quality-driven approach provides appropriate accuracy for individual patient QA, the procedure-time driven approach can only be used for preliminary measurements.     

低剂量螺旋CT扫描对肺癌模拟定位应用价值

目的 探讨低剂量螺旋CT扫描在肺癌模拟定位的可行性。
方法 选择确诊为肺癌患者62例。在电压120 kV、层厚5 mm、螺距1∶1、扫描时间1 s条件下按20、100 mA依次扫描2次,按优、良、差评价图像质量。利用治疗计划系统测定肿瘤靶区,记录单次扫描加权剂量指数、z轴扫描范围、剂量长度乘积用于估算患者受照剂量。对CT图像质量和肿瘤大小、组间剂量差异分别行Fisher′s精确概率法和配对t检验。
结果 低剂量与高剂量扫描的图像质量相似(优、良、差分别为46、13、3例与50、11、1例,P=0.541),同一肿瘤的靶区大小也相似(36.78、40.35 cm³,t=2.57,P=0.189),低剂量扫描剂量远小于高剂量的(133.05、941.25 mGy,t=－41.24,P=0.000)。
结论 采用20 mA管电流螺旋CT模拟定位扫描能基本保证靶区勾画和扫描图像质量,明显减少照射剂量对患者造成的可能伤害。     

Dosimetric validation of the MCNPX Monte Carlo simulation for radiobiologic studies of megavoltage grid radiotherapy

PURPOSE: To validate the MCNPX Monte Carlo simulation for radiobiologic studies of megavoltage grid radiotherapy. METHODS AND MATERIALS: EDR2 films, a scanning water phantom with microionization chamber and MCNPX Monte Carlo code, were used to study the dosimetric characteristics of a commercially available megavoltage grid therapy collimator. The measured dose profiles, ratios between maximum and minimum doses at 1.5 cm depth, and percentage depth dose curve were compared with those obtained in the simulations. The simulated two-dimensional dose profile and the linear-quadratic formalism of cell survival were used to calculate survival statistics of tumor and normal cells for the treatment of melanoma with a list of doses of the fractionated grid therapy. RESULTS: A good agreement between the simulated and measured dose data was found. The therapeutic ratio based on normal cell survival has been defined and calculated for treating both the acute and late responding melanoma tumors. The grid therapy in this study was found to be advantageous for treating the acutely responding tumors, but not for late responding tumors. CONCLUSIONS: Monte Carlo technique was demonstrated to be able to provide the dosimetric characteristics for grid therapy. The therapeutic ratio was dependent not only on the single alpha/beta value, but also on the individual alpha and beta values. Acutely responding tumors and radiosensitive normal tissues are more suitable for using the grid therapy.     

剂量分布验证中分辨率对Gamma通过率的影响

目的 探讨调强放疗验证中参考分布与被评估分布的分辨率对Gamma分析结果的影响。方法 设计4种不同类型的射野,使用TPS在水体模中计算并导出剂量分布的分辨率依次为1、2、3、4、5、6 mm。对不同取样分辨率采用IBA公司OmniPro-I′mRT软件计算Gamma指数。结果 当被评估分辨率一定时,参考分辨率的改变对Gamma通过率影响很小(调强射野5%以内);当参考分辨率一定时,Gamma通过率随被评估分辨率>1 mm范围内升高有升高趋\[从6 mm升高至3 mm通过率升高(15.2±6.2)%(t=11.99,P<0.01),从3 mm升高至1 mm通过率升高(14.9±5.5)%(t=13.24,P<0.01)\]。结论 应用Gamma分析方法进行调强放疗剂量分布比较时,可将测量数据作为参考分布,无须插值;将治疗计划系统计算数据作为被评估分布,其分辨率选取1 mm较合适。     

Dosimetric verification of compensated beams using radiographic film

Introduction

External photon beam modulation using compensators in order to achieve a desired dose distribution when brachytherapy treatment is followed by external beam radiation is a well-established technique. A compensator modulates the central part of the beam, and the dose beneath the thickest part of the compensator is delivered mostly by scattered, low energy photons. A two-dimensional detector with a good spatial resolution is needed for the verification of those beams. In this work, the influence of different types of detectors on the measured modulated dose distributions was examined.

Materials and methods

Dosimetric verification was performed using X-Omat V, Eastman Kodak radiographic films at different depths in a solid water phantom. The film measurements were compared with those made by ionization chambers. Photon beams were also modelled using EGSnrc Monte Carlo algorithm to explain the measured results.

Results

Monte Carlo calculated over-response of the film under the thickest part of the compensator was over 15%, which was confirmed by measurements. The magnitude of over-response could be associated with changes in the spectra of photon energy in the beam.

Conclusions

The radiographic film can be used for the dosimetry of compensated high energy photon beams, with limitations in volumes where photon spectra are hardly degraded.     

基于霍夫变换的射波刀等中心误差胶片分析算法

目的 提出一种基于霍夫变换(HT)的新算法提高射波刀自动质量保证(AQA)测试胶片影像分析的准确性与稳定性,并探讨胶片图像的扫描分辨率对测试结果的影响。方法 获取9对胶片对AQA模体进行分析测试,首先利用中值滤波对灰度化后的胶片影像预处理,去除噪声干扰;再使用全局阈值对图像进行二值化分割,对分割后的图像进行边缘检测并利用HT提取胶片边缘直线,将胶片图像变换至正确位置;最后利用边缘检测和HT提取出影像中射野投影圆和钨球投影圆的圆心,通过分析同心度最终得到AQA测试的误差分析结果。结果 所有样本分别使用本算法与原软件算法所得等中心误差结果差异无统计学意义(P>0.05),其标准差差异有统计学意义(P=0.027);表明本算法在保证了胶片分析准确性的同时具有更好的稳定性。两种方法均不能通过采用分辨率更高的胶片影像来提高分析结果的准确性和稳定性。结论 本算法排除了胶片扫描摆位误差造成的干扰,为射波刀系统AQA工作提供了一种更稳定的途径。     

Toward automated quality assurance for intensity- modulated radiation therapy

PURPOSE: To investigate whether high-quality, relatively inexpensive, document and transparency scanners used as densitometers are sufficiently quantitative for routine quality assurance (QA). METHODS AND MATERIALS: The scanner we investigated used a linear amplifier, digitizing gray-scale images to 12-bit resolution with a user-selected spatial resolution of 0.170 mm(2) pixels. To reduce Newton's rings artifacts, the standard glass platen was replaced by glass with an antireflective coating. Conversion of reading to transmission was conducted by permanently placing a calibrated photographic step tablet on the scanner platen. After conversion to light transmission, a zero-phase two-dimensional Wiener filter was used to reduce pixel-to-pixel signal variation. Light-scatter artifacts were removed by deconvolution of a measured light-spread kernel. The light-spread kernel artifacts were significant along the scanner's detector axis, but were insignificant along the scanning axis. RESULTS: Pixel-to-pixel noise was better than 2% for optical densities, ranging from 0.4 to 2.0 and 0 to 2.7 for the unfiltered and filtered images, respectively. The document scanning system response was compared against a confocal scanning laser densitometer. A series of IMRT dose distribution and dose calibration film sets were scanned using the two scanners, and the measured dose was compared. The maximum mean and standard deviation of the measured dose difference between the document scanner and confocal scanner was 1.48% and 1.06%, respectively. CONCLUSION: While the document scanners are not as flexible as dedicated film densitometers, these results indicate that, using the intensity and scatter corrections, the system provides accurate and precise measurements up to an optical density of 2.0, sufficient for routine IMRT film QA. For some film types, this requires the reduction in monitor units to limit the dose delivered to the film. The user must be cautious that the delivered IMRT dose is scaled appropriately. This inexpensive and accurate system is being integrated into an automated QA program.     

Investigations of beta-dosimetry at two different sources for the cardiovascular brachytherapy

The intracoronary brachytherapy is used at the Hamburg University Hospital as a method to treat in-stent restenosis. Two different radiochromic film types were applied to obtain dosimetric information of the beta-sources used (32P and 90Sr/90Y). First, these films were analyzed for their suitability for dosimetry. Within the investigated dose range (MD-55-2: 0 to 33 Gy, HD-810: 0 to 105 Gy), both films showed a linear behavior between the dose and the optical density (OD). Because radiochromic films are subject to time-based changes in OD, a method for colour stabilization was investigated (RCS-method). This method allowed to greatly shorten the time between irradiation and evaluation from 24 hours (time necessary for the film to reach a quasi-stable status) to 2.5 hours. Colour-stabilized films can also be stored for a long time and reanalyzed with almost the same results. Within the limits of the measurements error, both film types showed an energy independent response. Within the dose profiles, analyses of the two source types resulted in differences of 13.5% (32P) and 21% (90Sr/90Y). These inhomogenities are consistent with the fabrication tolerances given by the manufactures.